The purpose of this project is to describe microbial populations of plaque associated with exposed root surfaces and to detect changes in these populations that are associated with the development of carious lesions. A further aim of the project is to develop a composite picture of the microbial population of root surface lesions from surface plaque to advancing front of decalcification. Two experimental approaches will be used. One will be the automated predominant cultivable flora technique developed at Forsyth for describing the flora of periodontal pockets. This will provide a complete microbiological profile for selected samples taken from intact cemented surfaces and active disease sites. The other experimental approach will involve a simplified and partially automated system for culturing samples on selective media designed to detect major groups of acidogenic bacteria likely to be involved in root surface caries. This approach will allow processing of larger numbers of samples. The purpose of this phase of the work is to detect any shifts in these major groups of organisms as the various population groups are examined. An important goal of the project is the coordination and correlation of microbiological data with parameters of disease recorded in the clinical phase of the proposal. The animal studies will determine whether strains of oral Actinomcyes and streptococci act additively, synergistically, or competitively in the formation of root surface carious lesions in gnotobiotic animals. Groups of germ-free rats will be infected with combinations of A. viscosus and certain Streptococcus species; control animals will be monoinfected with the respective organisms alone. The colonization of each organism on the teeth of the rats will be monitored, and the extent of root surface decay which develops will be determined. Other variables to be studied include the use of high sucrose and high starch diets, and streptococcal strains which do, or do not coaggregate with the Actinomcyes. The data obtained should provide insights as to the importance of interactions which may occur between oral streptococci and Actinomcyes species found in human root surface decay.